The development of herbicide tolerant crops allows for the greater use of post-emergent herbicides during agricultural cultivation of the crop. One example of a post-emergent herbicide is N-phosphonomethylglycine, also known as glyphosate, a well known herbicide that has activity on a broad spectrum of plant species. Glyphosate is the active ingredient of Roundup® (Monsanto Co., St. Louis, Mo.), a safe herbicide having a desirably short half-life in the environment. When applied onto a plant surface, glyphosate moves systemically through the plant. Glyphosate is toxic to plants by inhibiting an enzyme in the shikimic acid pathway that provides a precursor for the synthesis of aromatic amino acids. Plants, fungi and some bacteria contain the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) enzyme that is sensitive to the toxic effects of glyphosate.
Farmers typically rely on genetic resistance to provide protection from plant pathogen infection and disease. However, sufficient genetic resistance is not always available in the crops being produced or undesirable traits are linked to the genetic resistance genetic loci. Farmers must then apply pesticides to control the pathogen infections, significantly increasing the cost of growing the crops and impact to the environment.
Controlling the crop loss to fungal diseases is expensive. The United States Department of Agriculture estimated that fungicide use to combat the Asian soybean rust alone could add $25 an acre, or 15 percent to 20 percent, to the cost of growing soybeans. If fungicides were applied to all U.S. fields planted with soybeans in 2004, it would cost farmers a total of about $1.87 billion.
It would be advantageous to develop methods and chemical mixtures for controlling pathogens and disease in glyphosate tolerant crop plants using compositions that are effective and safe. Such methods would reduce the cost of growing crops by reducing the number of inputs a farmer uses to treat a crop field while providing protection from losses do to plant disease.